This application is based upon and claims the benefit of priority from the prior Japanese Patent Applications No. 2000-230923, filed Jul. 31, 2000; and No. 2001-216986, filed Jul. 17, 2001, the entire contents of both of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to a color cathode ray tube of a shadow-mask type and a mask frame used in the same.
2. Description of the Related Art
In a color cathode ray tube of a shadow-mask type that is presently in wide use, a shadow mask for use as a color sorting mechanism comprises a rectangular mask body formed having a large number of electron beam holes and a rectangular mask frame supporting the mask body. In order to display an image free from color drift on a phosphor screen, three electron beams having passed through the electron beam holes must be landed correctly on their corresponding three-color phosphor layers. To attain this, the shadow mask must be held in a correct position relative to a panel. The shadow mask can be removably supported inside the panel in a manner such that elastic supporters that attached to corner sidewalls of the mask frame are engaged with stud pins on a skirt portion of the panel.
In order to meet the requirement for higher resolution to cope with the modern development of multimedia systems, the arrangement pitch of three-color phosphor layers of a phosphor screen is shortened. Accordingly, the tolerance for beam landing is so small that color drift easily occurs. Thus, the beam landing requires higher accuracy.
On the other hand, as modern color cathode ray tubes become larger in size, because of an increasing demand, mask frames accordingly become heavier.
Further, consideration must be given to impact that acts on a color cathode ray tube during its manufacture or transportation. The color cathode ray tube is provided with elastic supporters that can absorb external impact applied thereto. If the impact resistance of the elastic supporters is increased so that the supporters can support the heavy mask frame without causing dislocation under the external impact, the mask frame may possibly be deformed, failing to stand force that acts thereon as the shadow mask is loaded or unloaded during the manufacture of the color cathode ray tube. If the mask frame is deformed, the shadow mask that is fixed with it may possibly be deformed and dislocated with respect to the panel.
Accordingly, the mask frame is expected to be enhanced in strength without gaining weight. As an example of a measure to meet this requirement, there is proposed a method in which recesses are formed ranging from the sidewall portion of the mask frame to the base. In order to improve the rigidity of a mask frame 1A against torsion around its diagonal axis (axis D), the mask frame 1A is provided with recesses 4 near its corners, which connect sidewalls 2 and a base 3, as shown in FIGS. 7A and 7B, for example.
In the mask frame structure shown in FIGS. 7A and 7B, however, each recess 4 is provided ranging from the sidewall 2 at each corner end portion to the base 3, so that the diagonal dimension of the mask frame 1A varies substantially from a desired value. If the diagonal dimension of the mask frame 1A is smaller than the desired value, then its variation prevents a mask body 1B from being held in the mask frame 1A when the mask body 1B is welded to the mask frame 1A. If the diagonal dimension of the mask frame 1A is larger than the desired value, then the mask body 1B is pulled and easily deformed during the mask welding operation.
These problems are particularly significant for a color cathode ray tube that is provided with a shadow mask having a substantially flat curved surface or a flat face panel of which the curvature radius of the outer surface is substantially infinite.
Described in Jpn. Pat. Appln. KOKAI Publication No. 6-44919, on the other hand, is a structure that is provided with an annular cross bead 8 in a base 7 of a mask frame 5 and longitudinal beads 9 that extend from the cross bead 8 toward a frame sidewall 6, as shown in FIG. 8.
In general, residual stress (strain) is generated in the material of a press-formed mask frame. Normally, this residual stress is removed in a heat treatment process. In the case where the bead 8 is formed surrounding the opening of the mask frame 5, as shown in FIG. 8, however, deformation may possibly be caused along the bead 8 as the residual stress is removed during the heat treatment process. This deformation influences the shapes of the sidewall and the opening of the mask frame 5. Long and short side portions of the sidewall are lower in rigidity than corner portions, and deformation of the sidewall results in deformation of the mask body.
If the opening of the mask frame 5 is deformed toward the phosphor screen or in a manner such that it practically narrows, then the electron beams are unduly blocked, so that non-luminous regions are formed inevitably. If the opening of the mask frame 5 is deformed toward the electron gun or in a manner such that is practically widens, on the other hand, then the electron beams are blocked inadequately, so that unnecessary light emission is caused.
The present invention has been contrived in consideration of these circumstances, and its object is to provide a mask frame, capable of maintaining torsional rigidity without increasing weight and of restraining deformation of a mask body, and a color cathode ray tube provided with the mask frame.
According to the present invention, there is provided a color cathode ray tube comprising: a vacuum envelope including a panel having a major axis and a minor axis perpendicular to each other; a phosphor screen located on the inner surface of the panel; an electron gun assembly for emitting electron beams toward the phosphor screen; and a color sorting mechanism opposed to the phosphor screen, the color sorting mechanism including a mask body, having a rectangular effective portion with a large number of electron beam holes, and a mask frame fixedly fitted with the mask body, the mask frame including a sidewall portion, composed of a pair of long sidewalls extending parallel to the major axis and opposed to each other, a pair of short sidewalls extending parallel to the minor axis and opposed to each other, and corner sidewalls between the long and short sidewalls, and a base portion extending from the sidewall portion toward a tube axis, the base portion having substantially straight beads located near the corner sidewalls and connecting the long and short sidewalls.
According to the present invention, moreover, there is provided a mask frame comprising: a sidewall portion in the form of a rectangular frame including a pair of long sidewalls opposed to each other, a pair of short sidewalls opposed to each other, and corner sidewalls between the long and short sidewalls; and a base portion extending from the sidewall portion toward the center of the rectangular frame, the base portion having substantially straight beads located near the corner sidewalls and connecting the long and short sidewalls.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.